GREENHOUSE STRUCTURES AND OPERATIONS (HEATING, COOLING & ENVIRONMENTAL CONTROL SYSTEMS)

R. Allen Straw Specialist

Virginia Cooperative Extension

GREENHOUSES

Hobbyist, spare time Supplemental income Full time income

The intended use of the structures

determine the type, size and quantities of greenhouses.

How is the structure to be used? Retail sales Sole structure Retail sales area/ greenhouse

Wholesale sales Based on market demand Contract grower

Educational Hobby Public display Research

Trace your market before deciding on structures

Transplant/ Liner

Propagators

Nursery Growers

Landscape Contractors

Residential Construction

Commercial Construction

Wholesale Customers

Retail Garden Centers

Commercial Small Fruit Producers

Increased Crop Inventory

New Varieties

Crop Renovation

Commercial Vegetable Producers

Field Inventory Needed Per

Crop

2nd and 3rd

Planting per Season

New Crops

Greenhouse Growers

Wholesale Customers

Retail Customers

Greenhouse Size

Length 96 ft. (120 – 144 ft.) Ideal

< 96 ft. Increases your expense

> 96 ft. Makes air movement and

ventilation difficult

Width Freestanding 30 – 32 ft.

Gutter Connect 24 ft. 30 – 32 ft.

Overall 24 ft. X 96 ft. (gutter) 32 ft. X 96 ft. (free)

Greenhouse Components

Structures and Framing Glazing End Walls Heating Cooling and Ventilation Systems Thermostats and Controls Floor Benches

Structures

Structures- Free Standing Quonset

Structures- Free Standing Gable or Gothic

Structures- Gutter Connect

Greenhouse Types - Homemade

Inexpensive Often used in soil

culture Difficult to heat Difficult to ventilate

Greenhouse Types - Quonset

Moderate expense Unusable production

space on edges Machinery utilization

difficult

Greenhouse Types - Short Sidewall

Moderate expense Less unusable space

on edges than Quonset houses

Equipment utilization (?) – dependant on height of sidewalls

Greenhouse Types - Sidewall

More expensive Load Rated structure 70 mph / 10 lb/ft2

80 mph / 20 lb/ft2

Long Life $20,000 plus

Greenhouse Types – Gutter Connect

More expensive

Load Rated structure 70 mph / 10 lb/ft2

80 mph / 20 lb/ft2

Long Life

$20,000 plus

Good for multiple houses

More efficient to heat

Structure Variations

BELOW GROUND Studebaker Nurseries, Lancaster, Ohio

Framing

Materials for framing: •Galvanized Steel •Rigid Conduit •Thin wall Conduit •Structural Aluminum •Wood •Other materials & combinations

Framing Materials

Galvanized Steel Extruded Aluminum

Galvanized Conduit

Greenhouse Coverings (Glazing)

Glass Fiberglass Polycarbonate Double wall plastic (Poly) Film Plastic

Glazing, ctd.

GLASS

Glazing, ctd.

POLYCARBONATE

Glazing, ctd.

Glazing (covering)

FILM PLASTIC- (single layer)

Glazing ctd.

DOUBLE POLY COVER (Inflated)

Inflation System for Double Poly

Greenhouse Plastic Inflation

Position to keep about 2 inches of air between the layers of plastic

Use outside air to reduce condensation

Use jumper hoses for end walls

Do not over inflate Cancels the effect of the

dead air space Makes the plastic more

prone to wind damage

End Walls

POLYCARBONATE

PLASTIC

WOOD

FIBERGLASS

Greenhouse Coverings

Plastic 4 mil. (no!) 6 mil. Single layer (no!) Double layer

Affordable

Fiberglass (no!) Yellows and reduces

light penetration

Polycarbonate Single layer (no!) Twin wall Triple wall Expensive Good for end walls

Glass Longevity Very Expensive!

Greenhouse Heating

Heat need equations Heat required = U x A x (Tinside – Toutside) Where U = heat flow coefficient Single layer of plastic = 1.2 Double layer of plastic = 0.8

A = surface area of the greenhouse T = temperature rise

Calculating Heating Needs Heating systems are rated in British thermal units

(Btu) per hour (h). The Btu capacity of the heating system equals Q

The total exposed (outside) area of the greenhouse sides, ends, and roof in square feet equals A

The heat loss factor that quantifies the rate at which heat energy flows out of the greenhouse is equal to u ( A single cover of plastic or glass has a u factor of 1.2 Btu/hr. Double poly is .8 and polycarbonate is .53)

(Ti-To) is the maximum temperature difference between the lowest outside temperature (To) and the temperature to be maintained inside (Ti)

The formula is Q =A x u x (Ti-To)

Example (Small GH)

A 16’ wide by 24 ‘ long, 12’ ridge, 6’ sides, single layer glass greenhouse.

A = 1056 ft2 exposed area

U = 1.2 heat loss factor Ti = 60 deg F To = 0 deg F

Q = 1056 x 1.2 x 60 = 76,032 Btu/hr Furnace Output

Heating Example (Larger GH)

Parameters U = 0.8 (double layer plastic) A = 4,600 ft2 (32 ft. x 96 ft.) T = 60 degree rise

Heat Requirement = 0.8 X 4,600 x 60 = 220,800 btu’s

80% efficiency = 276,000 btu’s Two (2) - 150,000 btu furnaces

Heat “Types”

Larger ranges with centralized heating Steam Hot Water Smaller ranges with individualized heat units Electric Natural Gas Propane Infrared vacuum units Wood / Coal

Heat, ctd.

Boilers

Hot Water Steam

Heat, ctd.

Hot Water Heat In Floor

Heat, ctd.

Electric

Heat, ctd.

Propane & Natural Gas

Heat, ctd.

Infrared vacuum unit

Wood / Coal

Calculating Ventilation Needs In the summer, a greenhouse needs a 1:1 air exchange per

minute- exchanging inside air with outside air. In the winter, it needs only 20-30% of this amount to avoid

chilling the plants. Cubic footage of the greenhouse can be estimated using

the floor area times the average height of the house (side height + ridge height /2)

Exhaust fans are rated at CFM’s (cubic feet per minute) The fan or fans need to push (exchange) the total cubic

footage of air from the greenhouse each minute. The reduced amount of air exchange for winter is

accomplished by using a two stage fan/ thermostat control.

Greenhouse Ventilation/Cooling

Minimum of 1 air exchange / minute

Calculate volume (ft.3) of the greenhouse 32 ft. x 96 ft. 30,000 ft.3

Two 15,000 cfm fans 48 in. fans

Inlets Ft.3/700 (wind velocity at

inlet in ft./min.) 30,000 / 700 = 43 ft.2

Two 5 ft. square shutters Rule of thumb Inlet shutters should be

1.25 to 1.5 times larger that fan dia.

Cooling and Ventilation Systems

Cooling Wet pad system Shade Cloth

Ventilation Motorized shutters Exhaust fans Horizontal Air Flow (HAF) Convection Tube system Mechanical side curtain Hydraulic Roof System

Cooling- Wet Pad System

Cooling- Wet Pad System

Cooling- Shade Cloth

Ventilation Layout

Ventilation

MOTORIZED SHUTTERS

EXHAUST FAN WITH SPRING SHUTTER

Ventilation

HORIZONTAL AIR FLOW FANS

Ventilation

CONVECTION TUBE

Ventilation

MECHANICAL SIDE CURTAINS

Ventilation

HYDRAULIC ROOF SYSTEM

Greenhouse Air Circulation

Horizontal air flow fans 12 to 18 in. 4 in the house Make a race track ¼ way in from side

walls ¼ to 1/3 way in from

endwalls

HAF and Heater Configuration

HAF and Heater Configuration

Humidity Control

Humidity control A full canopy of a

tomato or cucumber crops will produce significant amounts of moisture through transpiration

A closed GH maintains that moisture

Humidity control fan in the top of the house works very well.

Humidity Control (cont.)

As low as possible Optimum 60 – 70%

Realistic 80 – 90%

Humidity control fan Switch Timer Humidistat

Thermostats and Controls

NOTE: Make sure that any thermostat used is one that will register down to or preferably below freezing and above 90 deg F. Normal house thermostats only go down to 40 deg F.

Thermostats and Controls

NOTE: Greenhouse thermostats and controls should not be mounted on outside walls or in direct air currents.

Thermostats and Controls

Thermostats and Controls

Thermostats and Manual Controls

Floor

BARE GROUND

Floor

Woven Landscape Fabric

Floor

GEO TEXTILE/ GRAVEL

Floor

CONCRETE Note: Used brushed concrete to avoid slippage when wet. Floors have a crown to drain excess water.

Benches

WOOD FRAME/ HARDWARE CLOTH

Benches

PASSVIE SOLAR

Benches

HYDROPONIC SYSTEMS

Benches

ROLLING BENCH SYSTEMS

Greenhouse Growing Area

Goal should be to have more bench space/ plant growing area than walkways. Cost per plant will be related to populations of plant materials being grown.

Costs of Greenhouse Construction 30’x48’ Greenhouse Double Poly Glazing Inflator Fan Unit Polycarbonate end walls Modine Gas Heating Exhaust Fans Motorized Inlet Shutters HAF Fans (Wet Wall optional) (Benches optional)

$11995.00 Package $6800.00 Labor (57%) $18,795.00 Total

2010 Estimate 1440 Square feet

$13.05 per Sq. ft. Cost (Turnkey)

Is there a difference…

Questions ??

R. Allen Straw

Virginia Cooperative Extension

931.261.0973

astraw@vt.edu